This invention relates to a plasma treatment method and a plasma treatment system using high-frequency discharge.
In semiconductor manufacturing, the plasma treatment method using high-frequency discharge is used in various fields of dry etching for microfabrication, sputtering for thin film formation, plasma CVD, etc.
The dry etching appropriate for microfabrication will be discussed as an application example of the plasma treatment method.
The dry etching is a fabrication technique using a chemical or physical reaction between vapor phase and solid phase surface caused by radicals, ions, etc., existing in plasma to remove an unnecessary portion of a specimen, such as a thin film or a substrate.
Reactive ion etching (RIE) used most widely as the dry etching technique is to expose a specimen to high-frequency discharge plasma of proper gas, thereby causing an etching reaction for removing an unnecessary portion of the surface of the specimen. The necessary portion of the specimen, namely, the unremoved portion normally is protected by a photoresist pattern used as a mask.
Orientation of ions needs to be established to promote microminiaturization in the reactive ion etching. To do this, it is indispensible to reduce scattering of ions in plasma. To establish alignment of ions, it is effective to lower pressure in a plasma generator and enlarge an average free path of ions. However, if the pressure in the plasma generator is lowered, the radical density lowers and the etching speed slows down.
As countermeasures against the problem, high-density plasma systems such as an inductively coupled plasma system and a Helicon plasma system are introduced. The high-density plasma system can generate plasma at 10-100 times higher density as compared with a conventional parallel plate RIE system and can provide etching speed equal to or more than that of the parallel plate RIE system even under a condition wherein pressure is lower 1/10-1/100.
However, it became evident that problems such as 1) abnormal etching profile; 2) occurrence of microloading effect; and 3) occurrence of degradation or destruction of gate insulating film occur in the conventional plasma treatment method thus improved. This topic will be discussed in detail.
In the conventional plasma treatment method, the specimen treated with plasma is negatively biased due to self-bias. Such self-bias is caused by the fact that the specimen is negatively charged on average as time by radio frequency power (RF) applied to the specimen stage.
In the conventional plasma treatment method wherein the specimen is thus biased negatively, the surface of the specimen is charged up by positive ions and electrons incident on the specimen.
When the specimen is charged up, the etching profile becomes abnormal. For example, when etching a polycrystalline silicon substrate by plasma treatment and the specimen is charged as described above, the pattern side wall becomes negatively charged and the pattern bottom surface becomes positively charged. If such potential difference imbalance occurs, the incidence angle of positive ions on each line pattern scatters and the profile of the pattern sidewall does not become abrupt and becomes tapering, so that an abnormal etching form occurs.
Further, for example, considering similar etching to that described above, a line pattern adjoins the inner side face of the outermost line pattern in a plane view, but no line pattern adjoins the outer side face of the outermost line pattern. Because of such an uneven form, on the outermost line pattern, the charge amount becomes uneven inside and outside the pattern (a region where no pattern exists is defined as outside and a region where patterns exist continuously is defined as inside). Resultantly, the charge-up amount inside the pattern differs from that outside the pattern, causing a potential difference to occur. The incident ion amount varies because of the potential difference and wedge-like holes, generally called notches, are made in the inner side face of the pattern on which more ions are incident. This fact is also described in K. K Chi and others, DRY PROCESS SYMPOSIUM Yokoushuu, p.75, Denki gakkai 1995, for example.
Such localization and unevenness of ions also affect the etching speed itself. That is, a photo resist mask is positively charged because of incident positive ions during etching. Thus, the smaller (narrower) the mask opening, the stronger the incidence inhibition function of positive ions on the opening. Resultantly, the microloading effect of a phenomenon in which the smaller (narrower) the mask opening, the slower the etching speed is caused.
Further, to manufacture MOS devices using the conventional plasma treatment method causing such an uneven charge supply, gate insulating films are degraded and destroyed. That is, for example, to manufacture a MOS device having a gate insulating film of a very thin film about 10 nm or less thick, if an uneven charge supply is caused by the conventional plasma treatment method, the mutual conductance of the MOS device exposed to plasma is degraded, leading to electric breakdown in an extreme case. This fact is also described in ERIGUCHI and others, IEICE TRANCE. ELECTRON., VOL. B78-C261, the Institute of Electronics, Information and Communication Engineers, for example.
By the way, if the transistor size becomes 1 micron or less by microminiaturization, an LSI contains semiconductor devices having a so-called antenna structure where the wiring area is 10.sup.3 or 10.sup.4 times larger than the transistor area.
Since such an antenna structure promotes the above-described charge unevenness, degradation and destruction of gate insulating films caused by plasma together with microminiaturization may become important problems increasingly.
In addition, if the plasma density is raised by introducing the high density plasma system such as an inductively coupled plasma system or a Helicon plasma system, the charge-up amount also increases. Thus, such a problem must become more serious.
As one method for solving such a problem, pulse plasma process has been proposed (for example, Ohtake and others, DRY PROCESS SYMPOSIUM Yokoushuu, p.45, Denki gakkai 1995).
The pulse plasma process is to supply plasma generating radio frequency power (RF) like pulses and the existence of an off period in the radio frequency power supply moderates local charge accumulation. According to the method, electrons are decreased and negative ions occur in the off period of the radio frequency power supply, which also enhances charge distribution uniformity.
However, the pulse plasma process cannot sufficiently solve the problem of an uneven charge supply. That is, to use generated positive and negative ions to perform the etching operation in the example of Ohtake and others, a bias voltage at a frequency 600 kHz or less not causing a self-bias needs to be applied to a sample state on which a specimen is placed. However, if an AC bias voltage is applied to the specimen stage and plasma treatment (etching treatment) is executed even at such a low frequency, energy of ions flowing into the specimen becomes too large and the etching selectivity between the photo resist and the silicon substrate becomes only about 2; it is insufficient for highly accurate etching.